I’m sorry, but I genuinely think it was LAZINESS on IO’s part for not allowing the Master from Season 1 to carry into Season 2. People have claimed it’s because of a number of the newer features in Season 2 change the way you play Season 1… but that’s total shit.
There’s not much foliage to hide amongst during the Season 1 levels
I don’t really see to much difference in the crowds from Season 1
Mirrors - whoop tee doo… That doesn’t entirely change the gameplay in Season 1
Difficulty levels - Professional level in GOTY seems exactly the same as Mastery in Hitman 2
OMG - We get to bring a briefcase in Season 1??? WOO HOO…
On no - wait - we get to slap people with a fish in Season 1. They obviously had to re-write the code because of that. 
I know some will claim that even though things are different they won’t mind doing the challenges again, but chances are that none of these changes will probably affect how you complete the challenges again… you’ll probably approach them the SAME way you did in GOTY.
This is genuinely laziness on IO’s part… a mere pathetic attempt to pad out the game replayability.